7848
J. Am. Chem. Soc. 1998, 120, 7848-7859
Layer-by-Layer Assembly of Thin Film Zener Diodes from
Conducting Polymers and CdSe Nanoparticles
Thierry Cassagneau,† Thomas E. Mallouk,† and Janos H. Fendler*,‡
Contribution from the Department of Chemistry, 152 DaVey Laboratory, The PennsylVania State
UniVersity, UniVersity Park, PennsylVania 16802, and Center for AdVanced Materials Processing,
Clarkson UniVersity, P.O. Box 5814, Potsdam, New York 13699-5814
ReceiVed February 23, 1998
Abstract: Ultrathin films have been prepared by self-assembling trioctylphosphine oxide, TOPO, capped n-type
20-40 Å diameter CdSe nanoparticles and 1,6-hexadecanethiol, HDT, onto p-doped semiconducting polymers,
chemically deposited poly(3-methylthiophene), PMeT (for DeWice A), and electrochemically deposited poly(pyrrole), Ppy (for DeWice B). The semiconducting polymers have, in turn, been electrochemically layered
(for DeWice A) or layer-by-layer chemically assembled (for DeWice B) onto derivatized conducting substrates.
The ultrathin films have been characterized by absorption and emission spectroscopy, transmission electron
microscopy, scanning force microscopy, X-ray photoelectron spectroscopy, and by electrochemical measurements. By controlling the level of doping into the p-type junction, it was possible to prepare dissymmetrical
junctions and observe a rectifying behavior in the forward direction and a Zener breakdown in the reverse
direction for Au/PMeT/(HDT/CdSe)3, DeWice A, and for Au/MEA/Ppy/(HDT/CdSe)3, DeWice B. Additionally,
Au/MeA/PAH/CdSe (PAH ) poly(allylamine hydrochloride)), Au/MEA/Ppy/PSS/CdSe (PSS ) polystyrene
sulfonate), and Au/MEA/Ppy/R-ZrP/CdSe (R-ZrP ) R zirconium phosphate) films have been prepared and
characterized.
Introduction
Size quantization of semiconductor nanoparticles manifest
itself in markedly altered physical and chemical behavior.1-6
In particular, size dependent changes in mechanical, optical,
electrical, electrooptical, magnetic, and magnetooptical properties have been observed.7,8 Exploitation of the beneficial
properties of nanoparticles for device construction requires that
they are organized into high density two-dimensional arrays and/
or three-dimensional networks in which suitable electrical
contact can be made and in which the current, voltage, and/or
light intensity can be measured. The layer-by-layer selfassembly (adsorption)9 of oppositely charged polyelectrolytes,10
polyelectrolytes and nanoparticles,11,12a polyelectrolytes and
graphite oxide,12b polyelectrolytes and layered R-zirconium
†
The Pennsylvania State University.
Clarkson University.
(1) Fendler, J. H., Membrane Mimetic Approach to AdVanced Materials;
Springer-Verlag: Berlin, 1992.
(2) Fendler, J. H.; Meldrum, F. C. AdV. Mater. 1995, 7, 607.
(3) Alivisatos, A. P. Materials Research Society Bulletin 1995, XX, 23.
(4) Henglein, A. Ber. Bunseng. Phys. Chem. 1995, 99, 903.
(5) Hodes, G. Solar Energy Materials and Solar Cells 1994, 32, 323.
(6) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41.
(7) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370,
354.
(8) Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994,
244, 985.
(9) Decher, G. In ComprehensiVe Supramolecular Chemistry; Sauvage,
J.-P., Ed.; Pergamon Press: Oxford, 1996; pp 507-528.
(10) (a) Decher, G.; Hong, J.-D. Makromol. Chem. 1991, 46, 321. (b)
Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210/211, 504.
(c) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160. (d)
Lvov, Y.; Decher, G.; Môhwald, H. Langmuir 1993, 9, 481. (e) Lvov, Y.;
Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396. (f) Lvov,
Y.; Essler, F.; Decher, G. J. Phys. Chem. 1993, 97, 13773. (g) Schmitt, J.;
Grünewald, T.; Kjaer, K.; Pershan, P.; Decher, G.; Lösche, M. Macromolecules 1993, 26, 7058.
‡
phosphate nanoplatelets,13 and polyelectrolytes and clay platelets12,14 have provided an eminently suitable approach to the
construction of ultrathin films. That any number of layers of
nanoparticles (or platelets) of any composition can be adsorbed
in any desired order ensures the versatility of ultrathin film
construction by the self-assembly method.
The availability of appropriate semiconductor nanoparticles
and polyelectrolytes has opened the door to the construction of
a new class of electroluminescent devices7,15-18 including a
multicolor pixel voltage-controllable light-emitting diode.7 In
this approach, a rectifying p-n junction has been fabricated
from a p-type semiconducting polymer, SCP (poly(p-phenylene
(11) (a) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer,
R. E.; Shashidhar, R.; Calvert, J. M. AdV. Mater. 1997, 9, 61. (b) Ariga,
K.; Lvov, Y.; Onda, M.; Ichinose, I.; Kunitake, T. Chem. Lett. 1997, 125.
(c) Liu, Y.; Wang, A.; Claus, R. O. Appl. Phys. Lett. 1997, 71, 2265.
(12) (a) Kotov, N. A.; Dékány, I.; Fendler, J. H. J. Phys. Chem. 1995,
99, 13065. (b) Kotov, N. A.; Dékány, I.; Fendler, J. H. AdV. Mater. 1996,
8, 637. (c) Kotov, N. A.; Haraszti, T.; Turi, L.; Zavala, G.; Geer, R. E.;
Dékány, I.; Fendler, J. H. J. Am. Chem. Soc. 1997, 119, 6821.
(13) (a) Keller, S. W.; Kim, H.-Y.; Mallouk, T. E. J. Am. Chem. Soc.
1994, 116, 8816. (b) Keller, S. W.; Johnson, S. A.; Brigham, E. S.;
Yonemoto, E. H.; Mallouk, T. E. J. Am. Chem. Soc. 1995, 117, 12879. (c)
Kim, H.-N.; Keller, S. W.; Mallouk, T. E.; Smitt, J.; Decher, G. Chem.
Mater. 1997, 9, 1414.
(14) (a) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370. (b)
Kleinfeld, E. R.; Ferguson, G. S. AdV. Mater. 1995, 7, 414. (c) Kleinfeld,
E. R.; Ferguson, G. S. Mater. Res. Soc. Symp. 1995, 369, 697. (d) Kleinfeld,
E. S.; Ferguson, G. S. Chem. Mater. 1996, 8, 1575.
(15) (a) Dabbousi, B. O.; Murray, C. B.; Rubner, M. F.; Bawendi, M.
G. Chem. Mater. 1994, 6, 216. (b) Dabbousi, B. O.; Bawendi, M. G.;
Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316.
(16) (a) Colvin, V. L.; Alivisatos, A. P. Phys. ReV. Lett. 1991, 66, 2786.
(b) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc.
1992, 114, 5221.
(17) Gao, M.; Richter, B.; Kirstein, S. AdV. Mater. 1997, 9, 802.
(18) Kumar, N. D.; Joshi, M. P.; Friend, C. S.; Prasad, P. N.; Burzynski,
R. Appl. Phys. Lett. 1997, 71, 1388.
S0002-7863(98)00602-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/28/1998
Layer-by-Layer Assembly of Zener Diodes
J. Am. Chem. Soc., Vol. 120, No. 31, 1998 7849
vinylene), PPV, for example), layer and an n-type semiconductor
(cadmium selenide, CdSe, for example) nanoparticle layer.
These ultrathin films serving as p-n junctions have been
constructed by self-assembly (alternating layers of CdSe nanoparticles stabilized by 1,6-hexanedithiol and PPV),7 by the
Langmuir-Blodgett technique (transferring the CdSe nanoparticles to glass slides coated with a layer of sulfonated
polystyrene),15a and by spin-casting (after mixing the nanoclusters with poly(vinylcarbazole) as a hole conducting polymer and
an oxadiazole derivative, t-Bu-PBD as the electron transport
agent).15b
In addition to the rectifying behavior expected for such a
Schottky-type junction, one can also design, in principle, a Zener
junction by creating an asymmetry in the degree of doping of
the electroactive layers. For example, one can envisage to
control, chemically or electrochemically, the degree of p-doping
in the SCP without affecting the properties of the n-type
semiconductor nanoparticle layer. Such a Zener junction cannot
be constructed exclusively from semiconducting polymers
because p- and n-types of doping cannot be separately introduced to neighboring layers. Additionally, n-doped SCP
(polypyrrole or polythiophene, for example) tend to become
dedoped upon exposure to air.19 This problem can be overcome
by coupling the p-type SCP to an n-type semiconductor
nanoparticle layer.
Encouraged by the reported successful construction of nanoparticle based electroluminescence devices7,15,16 we have undertaken a systematic investigation of the assembling and selfassembling advanced electronic devices. Here, we report the
results of our study on two p-n junction based devices (DeWice
A and DeWice B) which are both rectifying and can undergo a
Zener breakdown. The p-type layer was constructed from SCPs,
using poly(3-methylthiophene) for DeWice A and poly(pyrrole)
for DeWice B. Self-assembled arrays of CdSe nanoparticles
constituted the n-type layer for both DeWice A and DeWice B.
Very similar behavior was observed for the two devices upon
the doping of the SCP. Applying a positive forward bias caused
rectification which followed the Fowler-Nordheim law, characteristic of charge injection by tunneling mechanism. Upon
applying a negative bias a Zener breakdown occurred which
was attributed to dissymmetrical doping of the n-type (the selfassembled arrays of CdSe nanoparticles) and the p-type (the
SCP) layers assembled in the ultrathin film.
purchased from Delta Technologies, White Bear Lake, MN. Gold
substrates were purchased from EMF Corp. (Ithaca, NY) and consisted
of a 1000 Å gold layer deposited onto a glass covered with a thin layer
of titanium (50 Å). These substrates were exposed to a fresh piranha
solution (75% H2SO4/25% H2O2) for 5-10 min and were copiously
washed with deionized water prior to using them. R Zirconium
phosphate, R-Zr(H2PO4)2‚H2O (R-ZrP) has been prepared by the direct
precipitation method.20a The colloidal suspension used in self-assembly
resulted from the exfoliation of R-ZrP in an aqueous solution of
propylamine as previously reported.20b
Instrumentation. Absorption spectra were recorded on a HewlettPackard 8452A diode array spectrophotometer. Ellipsometric data were
acquired by using a Gaertner Model L2W26D ellipsometer with a HeNe
laser (632.8 nm) light source. For measurements, the real part of the
refractive index was fixed to 1.54, while the imaginary part was
assumed to be zero. Thicknesses data were averaged values obtained
from different spots (5-6) on a given sample. Transmission electron
microscopy images were obtained by using a JEOL 1200 EXII
microscope operating at 120 kV. X-ray photoelectron spectra (XPS)
were determined by a Hewlett-Packard 5950A instrument with 256
multichannel resistor plates. Spectra were taken using the Al Ka line
source at 1486.6 eV. Approximate compositions were obtained from
simple curve fits of the Se 3s/s 2s envelope. Interference between the
S 2p peak (the most intense photoelectron peak for S) and other peaks
(Se 3p and Al X-ray ghosts from the Cd 3d peak, which were
comparable in size and very close to it) prevented the use of the
conventional fitting. Unfortunately, the uncertainty in the S concentration was as high as (20%, making an evaluation difficult. Additional
carbon was present due to storing samples in air for 20 months.
Therefore, the XPS results reported here should be considered qualitative. Atomic force microscopic (AFM) was performed with a Digital
Nanoscope IIIa system. The pyramidal AFM tips and cantilevers were
made from etched silicon probes. All AFM images were collected in
tapping mode in air, resonating the tip just below the oscillation
frequency of the cantilever (typically 250-325 kHz). The scanning
frequency was 1-2 Hz, and the data collection resolution was 512 ×
512 pixels. Tips were changed frequently to maintain the resolution
of the AFM images. Electrochemical syntheses were performed under
Ar atmosphere by using a Princeton Applied Research Model 273
potentiostat. The working electrode consisted of a gold (or ITO)
substrate while auxiliary and reference electrodes were platinum and
Ag/Ag+ respectively. i-V characteristics were obtained in air, at room
temperature, after evaporation of an Al film (several hundreds of Å)
on multilayers deposited onto gold substrates, by using a potentiostat
(same model) interfaced with a MacLab electrochemical software
version 1.3. Fluorescence spectra were obtained with a Spex Fluorolog
1681 fluorometer at room temperature using front face observations.
Experimental Section
Methods
Materials. Dimethylcadmium (99+%, Strem), 1,6-hexanedithiol
(>97%, Fluka, HDT), HCl (36.5-38%, Baker), and (4-aminobutyl)dimethylmethoxysilane (United Chemical Technologies, Inc., AMS)
were used as received. Poly(styrene-4-sulfonic acid), sodium salt, PSS,
20% solution in water, was obtained from Aldrich; before use it was
diluted to 0.02 M concentration with 0.01 M HCl. Pyrrole (98%),
selenium powder (100 mesh, 99.999%), trioctylphosphine (90%, TOP),
trioctylphosphine oxide (99%, TOPO), tetrabutylammonium tetrafluoroborate (99%), 2-aminoethanethiol hydrochloride (98%), 3-methylthiophene (98%), FeCl3‚6H2O (98%), anhydrous ethyl alcohol, anhydrous butanol (99.5%), anhydrous benzonitrile (99+%), anhydrous
powder of lithium tetrafluoroborate (99.999%), Rhodamine B, poly(allylamine hydrochloride) (Mw ) 50 000-65 000), and mercaptoethylamine hydrochloride (98%, MEA) were purchased from Aldrich
and used without further purification. Distilled water was put through
a Barnstead Nanopure II column to obtain deionized water with a
resistivity of 18 MW‚cm. Quartz slides was obtained from Chemglass,
Inc., and conducting Indium Tin Oxide, ITO, coated glass was
Substrate Derivatization. Both ITO and gold substrates
were pretreated by an appropriate molecule which could act as
anchoring agent for the semiconducting polymer by virtue of
their pH sensitive terminal amino groups. ITO substrates were
pretreated by (i) overnight (ca. 15 h) immersion into a 0.3 M
solution of (4-aminobutyl)dimethylmethoxysilane in toluene (in
a desiccator) and (ii) washing with toluene, ethanol, and water
to provide an 8 ( 1 Å thick (determined by ellipsometry)
coating. Gold substrates were pretreated by (i) immersion into
a 2% (w/v) aqueous 2-mercaptoethylamine hydrochloride solution for 12-15 h, (ii) washing by and sonicating in ethanol,
and (iii) washing with water to provide an 8 ( 1 Å thick
(determined by ellipsometry) coating.
Preparation of Polypyrrole, Ppy. Pyrrole monomer can be
oxidized at ca. +0.6-0.8 V vs SCE, thereby initiating the
formation of poly(pyrrole). The polymer is made in the oxidized
(19) (a) de Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand,
R. E. F. Synthetic Metals 1997, 87, 53. (b) Chowdhury, A.-N.; Harima, Y.;
Kunugi, Y.; Yamashita, K. Electrochim. Acta 1996, 41, 1993.
(20) (a) Alberti, G.; Torracca, E. J. Inorg. Nucl. Chem. 1968, 30, 317.
(b) Alberti, G.; Casciola, M.; Costantino, U. J. Colloid Interface Sci. 1985,
107, 256.
7850 J. Am. Chem. Soc., Vol. 120, No. 31, 1998
form and can be reduced at potentials negative of 0.0 V vs SCE.
It is reoxidized at potentials positive of about +0.2 V vs SCE.21
A poly(pyrrole) layer was formed by the polymerization of
pyrrole (0.02 M at pH ) 1.0, HCl) using acidic 0.006 M of
FeCl3‚6H2O as initiator. After aging for 20 min, the solution
was filtered through a fritted funnel keeping most of the black
pyrrole. The doping of the Ppy layer is controlled by the
oxidizing agent-to-pyrrole ratio. It has been characterized by
the absorption spectrophotometry prior to filtration22,23 which
revealed an intense interband absorption band at 382 nm (3.25
eV) characteristic of a πfπ* transition of the π electrons in
the highest occupied molecular orbitals and a weaker adsorption
band at 460 nm (2.70 eV) attributed to the presence of short
cationic conjugated segments.23 The extent of doping was found
to be time-dependent and was monitored spectroscopically by
plotting the ratios of absorbances at 460 nm to 382 nm against
time. Generally, solution aged for 20 min exhibited an
absorption ratio of about 2.7, indicating that about 40% of the
Ppy had been effectively doped.
Preparation of Poly(3-methylthiophene), PMeT. PMeT
films were electrochemically synthesized. For deposition onto
the pretreated ITO substrates an electrolytic medium consisting
of Li+BF4-(0.5 M) and monomeric 3-methylthiophene (0.4 M)
in benzonitrile was used.24 For deposition onto pretreated gold
substrates an electrolytic bath containing tetrabutylammoniumtetrafluoroborate, TBA+‚BF4- (0.1 M), and monomeric
3-methylthiophene (0.01 M) in HPLC acetonitrile was used.25
Prior to use the monomeric 3-methylthiophene solution was
degassed under vacuum below its boiling point for 20 min and
then transferred to a drybox where the electrolytic bath had been
prepared in a Schlenk flask. The degree of oxidation of PMeT
was estimated to be 0.12 charge per monomer unit.25b
Deposition of the Ppy Layer onto the Pretreated Substrates. Ppy layers were deposited by repeated dipping of the
pretreated substrate into the filtered Ppy solution, typically for
a period of 2 min, and washing extensively with deionized water.
The dipping and washing sequence was repeated until the
desired thickness was reached. The dipping time was increased
(by a factor of 10-15) when more qualitative filtration was
performed without improving the stability of the resulting Ppy
solution. In a typical procedure, precoated substrates were
immersed into aged (typically for 20 min) and filtered Ppy
solutions for two minutes for five times and washed by
deionized water between each times. Such treatment led to
100-200 Å thick Ppy layers. The RMS value for the surface
roughness of a 80 Å thick Ppy layer, deposited onto a precoated
gold substrate, was determined to be 18 Å.
Deposition of the PMeT Layer onto the Pretreated
Substrates. The deposition of a doped PMeT layer was carried
out in a Schlenk flask electrochemically. A potential of 10 V
was applied across the pretreated ITO working electrode (the
substrate), a platinum counter electrode, and a silver reference
(21) (a) Kanazawa, K. K.; Diaz, A. F.; Geiss, R. H.; Gill, W. D.; Kwak,
J. F.; Logan, J. A.; Rabolt, J. F.; Street, G. B. J. Chem. Soc., Chem. Commun.
1979, 854. (b) Diaz, A. F.; Martinez, A.; Kanazawa, K. K. J. Electroanal.
Chem. 1981, 130, 181. (c) Bull, R. A.; Fan, F. R.; Bard, A. J. J. Electrochem.
Soc. 1982, 129, 1009.
(22) Brédas, J. L.; Scott, J. C.; Yakushi, K.; Street, G. B. Phys. ReV. B
1984, 30, 1023.
(23) Yakashi, K.; Lauchlan, L. J.; Clarke, T. C.; Street, G. B. J. Chem.
Phys. 1983, 79, 4774.
(24) (a) Kaneto, K.; Yoshino, K.; Inuishi, Y. Jpn. J. Appl. Phys. 1983,
22, L567. (b) Kaneto, K.; Kohno, K.; Inuishi, Y. J. Chem. Soc., Chem.
Commun. 1983, 382.
(25) (a) Tourillon, G.; Garnier, F. J. Electroanal. Chem. 1982, 135, 173.
(b) Waltman, R. J.; Bargon, J.; Diaz, A. F. J. Phys. Chem. 1983, 87, 1459.
(c) Tourillon, G.; Garnier, F. J. Electroanal. Chem. 1984, 161, 51.
Cassagneau et al.
electrode in the appropriate electrolytic medium under an argon
atmosphere. A potential of 1.1 V was applied across the
pretreated gold working electrode (the substrate), a platinum
counter electrode, and a silver reference electrode in the
appropriate electrolytic medium under an argon atmosphere. In
both cases the film thickness was controlled by varying the
amounts of charge responsible for the polymerization. Because
the oxidation potential at which the polymerization of oligomers
occurs is lower than that due to the conversion of the monomer,
the obtained films were directly in a p-type conducting state as
characterized by their deep blue color.26
Synthesis of CdSe Nanocrystals. CdSe nanoparticles were
prepared by the published procedure.27,28 Briefly, 0.0035 mol
of dimethylcadmium was first added to 0.0025 mol of Se
dissolved in 20 mL of trioctylphosphine (TOP) in an Ar-filled
drybox. The mixture was transferred to a syringe and injected
into pure melted trioctylphosphine oxide (TOPO), previously
degassed under vacuum at 310-350 °C, under vigorous stirring
at the same temperature and under argon (1 atm). Purification
and size-selective precipitations were carried out three times in
ethanol, followed by redispersion in butanol.27 The synthesized
nearly monodisperse suspension contained TOPO capped (hence
the positive charge) CdSe nanocrystals of 35-37 Å in diameter,
as deduced from visible spectroscopy and ellipsometric measurements.
Self-Assembly Procedure. Multilayers of CdSe nanocrystallites were prepared by using 1,6-hexanedithiol as anchoring
molecule.7,16 The layering procedure consisted of dipping
successively the SCP covered pretreated substrate into (i) an
aqueous 5 mM 1,6-hexanedithiol solution for at least 12 h and
(ii) into a CdSe nanoparticle dispersion in butanol (approximately 4 × 10-3 M) for 7-8 h. The thiolated gold
substrate was washed with deionized water for 20 s and then
ethanol also for 20 s. After dipping in the CdSe suspension
the substrate was thoroughly washed with butanol (30 s) and
ethanol (20 s). In some cases, negatively charged organic (PSS)
and inorganic (R-ZrP) polyelectrolytes were self-assembled onto
the SCP covered pretreated substrate (Au/MEA/Ppy) prior to
the self-assembly of CdSe. This self-assembly was accomplished by dipping the Au/MEA/Ppy film into a 0.02 M PSS
solution (pH ) 1.0), for 10 h, followed by washing with
deionized water (20 s) and drying under an Ar gas stream, or a
diluted colloidal suspension (6 × 10-4 M) of R-ZrP for 5 s,
followed by a washing with deionized water (30 s) and ethanol
(20 s) and drying via Ar gas. Ellipsometric measurements
indicated that a PSS layer of 6.5 Å ( 2 Å and an R-ZrP film of
10 Å ( 1 Å (consistent with the value expected for a monolayer
of sheet, thickness 6.5 Å) were formed onto Au/MEA/Ppy films.
All the film-supported substrates were stored in air.
Results and Discussion
Strategy of Device Construction. Layer-by-layer assembly
and self-assembly of a versatile rectifying device capable of
undergoing a Zener breakdown has been the objective of the
present work. Our interest has been prompted by the possibility
of developing an ultrathin flat-panel-display screen based on a
phosphor29 whose electroluminescence can be triggered by “hot”
electrons produced by the Zener breakdown at an p-n type
(26) Tourillon, G. In Handbook of Conducting Polymers; Skotheim, T.
A., Ed.; Marcel Dekker: New York, 1986; pp 293-350.
(27) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc.
1993, 115, 8706.
(28) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468.
(29) Rack, P. D.; Naman, A.; Holloway, P. H.; Sun, S.-S.; Tuenge, R.
T. Mater. Res. Soc. Bulletin 1996, XXI, 49.
Layer-by-Layer Assembly of Zener Diodes
J. Am. Chem. Soc., Vol. 120, No. 31, 1998 7851
Figure 2. Schematic energy level diagram for a p+-n junction
consisting of a p-doped semiconductor polymer (poly(pyrrole), Ppy,
or poly(3-methylthiophene), PMeT) and CdSe nanoparticles.
Figure 1. Fabrication of p+-n junction by layer-by-layer assembly
and self-assembly, capable of functioning as a rectifying device
undergoing a Zener breakdown. The device consists of two layers: the
first layer is a p-doped semiconductor polymer, SCP (polypyrrole, Ppy,
for example), deposited onto a pretreated (by mercaptoethylamine
hydrochloride, MEA) conducting substrate (gold, for example) which
serves as the anode. The second layer is composed of n-type of
semiconductor nanoparticles (cadmium selenide, CdSe, for example)
and 1,6-hexanediol, HDT, layer-by-layer self-assembled onto the first
layer and coated by a thin film of aluminum (Al) which serves as the
cathode.
junction. The device fabricated consisted two layers. The first
layer contained a p-doped semiconductor polymer, SCP (polypyrrole, Ppy, for example), of variable thickness, deposited onto
a pretreated (by mercaptoethylamine hydrochloride, MEA)
conducting substrate (gold, for example) which served as the
anode. The second layer consisted of n-type of semiconductor
nanoparticles (cadmium selenide, CdSe) and 1,6-hexanedithiol,
HDT, layer-by-layer self-assembled onto the first layer and
coated by a thin film of aluminum (Al) which served as the
cathode (see Figure 1). Controlling the Zener breakdown by
appropriate doping the SCP layer has been pivotal to our
approach. Our design strategy has involved the selection of
the most suitable SCP and semiconductor nanoparticles by
considering their energy levels, the optimization of the composition and the thickness of each layer, and the doping of the SCP
layer.
Selection of SCP and Semiconductor Nanoparticles.
Components of the device have been selected by considering
their charge injection energy levels. An appropriate matching
must exist between the ionization potential (HOMO) of the
p-type SCP and the anode, between the electron affinity of the
n-type semiconductor nanoparticles and the cathode, and
between the SCP and the semiconducting nanoparticles. The
effect of thiolated molecules (1,6-hexanedithiol, HDT, and
mercaptoethylamine, MEA) on the electronic characteristics of
the device has been neglected because of the very high electron
affinity and band gap of these molecules. Energy diagrams of
the systems investigated in the present work is illustrated in
Figure 2.
CdSe nanocrystallites have an electroaffinity of 3.00 eV7,30
and a band gap of 2.10 eV7 quite comparable to those of poly(3-methylthiophene), PMeT (2.84 and 2.10 eV, respectively31).
It is known that when PMeT is p-doped at less than 30%, the
bipolaron levels appear inside the band gap at about 0.7 eV
(30) Colvin, V. L.; Alivisatos, A. P.; Tobin, J. G. Phys. ReV. Lett. 1991,
66, 2786.
(31) Yoshino, K.; Onoda, M.; Manda, Y.; Yokoyama, M. Jpn. J. Appl.
Phys. 1988, 27, L1606.
from the valence band and conduction band edges. In that case,
the bipolaron level closer to the valence band edge participates
in the activity of the p-junction when hole injection occurs in
the vicinity of this level (i.e., for gold substrates, for example).
The polaron level of Ppy, 5.19 eV (relative to the vacuum level),
is at a range comparable to that required for exciton formation
from CdSe; an energy difference of 2.19 eV separates the two
levels of CdSe and Ppy; as compared to a difference of 1.94
eV between CdSe and PMeT (see Figure 2).
These considerations led us to focus our attention on two
systems: Au/PMeT/(HDT/CdSe)3 (Device A) and Au/Ppy/
(HDT/CdSe)3 (System B). Details on the composition of these
two systems and on others examined are summarized in Table
1.
Optimization of the PMeT Layer. The electrochemical
deposition of a semiconducting thin film onto a conductive
substrate is expected to produce a very rough surface. Surface
roughness was decreased by the use of such anchoring molecules
as mercaptoethylamine, MEA (with Au substrate) and (4aminobutyl)dimethylmethoxysilane (with ITO coated glass
substrates). Surface roughness, evaluated by AFM, indicated
a typical RMS value of 110 Å for an electrochemically deposited
500-600 Å thick film. The anchoring molecules, by virtue of
providing a uniform monolayer coverage, also improved the
homogeneity of the subsequent semiconductor nanoparticle
layer.
Optimization of the Ppy Layer. Ppy binds irreversibly to
the MEA-derivatized surface, presumably through nucleophilic
attack of the amino group on the oxidized aromatic polymer.
The Ppy solution contains positively charged oligomers, dispersed by electrostatic repulsions, and less doped oligomers,
often referred to as black pyrrole, black-Ppy, which tend to
aggregate on aging. Ppy solutions were always freshly prepared
and used within a couple of hours. Filtration of the black Ppy
and repeated washing were found to be necessary to obtain good
quality Ppy films. Repeated washing of the substrates considerably enhanced the deposition rate for a given dipping time. The
reformation of black Ppy after filtration slows down the kinetic
of growth of the doped oligomers in the neighborhood of the
substrate by a “screening effect”. At a certain threshold,
depending upon the pH and on the deposition time, the
concentration of the aggregates became large enough for
inducing an irreversible deposition onto the substrate and
inhibiting further deposition of doped oligomers (Figure 3). TEM
picture of a Ppy film, prepared by dipping the substrate into a
Ppy solution for 20 min (and unwashed) taken on a derivatized
copper grid, evidenced the presence of ellipsoidal particles of
500 ( 80 Å in height in good accord with values measurement
by ellipsometry (630 Å ( 10 Å). At this stage, passivation of
the surface reactivity was indirectly observed, both by visible
spectroscopy and ellipsometry; attempts to deposit Ppy from
7852 J. Am. Chem. Soc., Vol. 120, No. 31, 1998
Cassagneau et al.
Table 1
type of interactiona
initial derivatized
substrate
covalent
binding
electrostatic
interaction
selfassembly
electrochemistry
solvent(s)
thickness of
one CdSe layerb
no
yes
yes
yesc
ethanol
butanol
35 Å
Device B
nof
yes
no
yes
yes
yes
yes
no
no
no
ethanol
butanol
water
water
butanol
35 Å
20 Åg,h
25-26 Åh
17-20 Åh
Vo.c. (V)
Device A
Au/PMeT/
Au/PMeT/HDT/
Au/MEA/Ppy/
Au/MEA/Ppy/HDT/
Au/MEA/PAH/
Au/MEA/Ppy/PSS/
Au/MEA/Ppy/R-ZrP/
between
(PMeT/HDT)1
(HDT/CdSe)n
between
(Ppy/HDT)1
(HDT/CdSe)n
(PAH/CdSe)1
(PSS/CdSe)1
(R-ZrP/CdSe)1
+1.0
+0.6d
+1.2-1.4e
+0.47
+0.45
a
Interactions between the two outermost layers as indicated by the brackets. b The diameter of CdSe nanoclusters was about 37 Å. c Although
CdSe can be deposited under a potential, the quality of the layer (uniformity, coverage) is not good enough. d Prior redoping. e After electrochemical
redoping. f Including in acetonitrile, dioxane, N-methylformamide, THF, DMF, or neat HDT. g This layer is likely physisorbed and can be removed
upon washing with water. h Further dipping in a capped CdSe butanol solution did not lead to additional deposition of CdSe nanoclusters.
Figure 4. Correlation between the time of dipping of the pretreated
(by mercaptoethylamine hydrochloride, MEA) gold substrate into
filtered polypyrrole solution and the resulting thickness (Å) of a Ppy
layer.
Figure 3. TEM image of a poly(pyrrole), Ppy, thin film (50 Å)
prepared by dipping the substrate into a filtered solution of chemically
p-doped Ppy for 20 min (without subsequent rinsing). The aggregates
seen on the film are “black” pyrrole.
fresh solutions were unsuccessful. Attempts to lay down any
other components (polyelectrolyte or HDT) onto the Ppy film
also failed. This kind of passivation was attributed to the
presence of big black Ppy aggregates (Figure 3). The thickness
and the quality of the Ppy layers were found to depend on the
frequency of the dipping and washing steps. Similarly, the rate
of deposition was found to depend on the dipping time and the
frequency of washing (see Figure 4); for example, it varied from
0.12 Å per s (2-3 min of dipping, washing after each dipping)
to only 0.03 Å per s (10-15 min of dipping followed by
washing).
Optimization of the Self-Assembly. Self-assembly onto the
derivatized substrate was mediated by two types of interactions: covalent binding, in case of using thiolated molecules,
and electrostatic interactions, in case of using organic and
inorganic polyelectrolytes. Stability in common solvents and
thicknesses of CdSe layers were routinely compared in order
to select the most suitable procedure for self-assembly. Gold
substrates were used for monitoring the thickness after each
step by ellipsometry, while ITO coated glass and quartz
substrates were chosen for studying the absorption properties
of the different layers.
For DeWice A (see Table 1) alternating layers of HDT and
CdSe were self-assembled onto an electrochemically deposited
PMeT layer. Self-assembly of HDT onto TOPO-capped CdSe,
resulted in the progressive displacement of the trioctylphosphine
shell by covalently bonded 1,6-hexanethiol molecules (Figure
5). This self-assembly, on a derivatized gold substrate (Au/
PMeT), was successfully repeated up to 10 sandwich layers (i.e.,
Au/PMeT(HDT/CdSe)10). In the present work we limited our
electrochemical studies to DeWice A which contained only three
sequences of HDT/CdSe (i.e., Au/PMeT(HDT/CdSe)3, see Table
1). Three sandwich layers of capped cadmium selenide, (HDT/
CdSe)3, self-assembled onto poly(phenyl-phenylene) has been
reported to be sufficient for the detection of electroactivity.7
For DeWice B (see Table 1) alternating layers of HDT and
CdSe were self-assembled onto a Ppy layer by taking advantage
of the covalent binding of HDT to CdSe. To obtain an
understanding of the interaction between HDT and the semiconducting polymer (Ppy) and between HDT and CdSe we
surveyed the effect of the solvents on the anchoring reaction.
Using an ethanolic solution as a solvent for HDT led to a
complete dissolution of the ultrathin Ppy layer, after a contact
time of 2-3 h only. However, depending on the concentration
of HDT and contacting time, a monolayer of HDT could be
anchored to the Ppy surface. The use of a nonprotic and polar
solvent such acetonitrile or dioxane was found to inhibit
interaction of HDT with the Ppy layer. Similarly, the use of
Layer-by-Layer Assembly of Zener Diodes
J. Am. Chem. Soc., Vol. 120, No. 31, 1998 7853
Figure 5. Simplified representation of the self-assembly involved in the layering of CdSe nanocrystallites, via electrostatic interactions (left) or
covalent binding (right).
N-methylformamide, THF, and DMF resulted in insufficient
binding of HDT. Using neat HDT also resulted in the complete
dissolution of the polymer ultrathin layer. This result was
somewhat unexpected in view of the well known formation of
thiol monolayers on gold substrates in ethanol, acetonitrile, THF
and DMSO.32 HDT could however be attached to Ppy
electrochemically (see below).
Additionally, alternating layers of polystyrene sulfonate, PSS
(or R-ZrP), and CdSe were self-assembled onto a Ppy layer by
taking advantage of attractive electrostatic interactions. In this
approach we have been guided by the successful demonstration
of ultrathin film formation between semiconductor nanoparticles
and oppositely charged polyelectrolytes and surfactants by
electrostatic interactions.12a,15a
Self-assembly by electrostatic interactions was investigated
first by using positively charged polyallylamine hydrochloride,
PAH, as the primer. Not surprisingly, dipping a derivatized
gold substrate which had been coated by a 3-4 Å thick PAH
layer (i.e., Au/MEA/PAH) into a CdSe dispersion for 15 h
resulted in the physisorption of a 20 Å thick layer of semiconductor nanoparticles, easily removed by rinsing by ethanol.
Conversely, negatively charged PSS and R-ZrP could be selfassembled onto the positively charged Ppy film in the Au/MEA/
Ppy system. Electrostatic attraction between Ppy and PSS (or
R-ZrP) was found to be, in fact, so strong that even sonication
in ethanol (20 min, in an ultrasonic cleaner) did not result in
the separation (or exfoliation) of the self-assembled Au/MEA/
Ppy/PSS or Au/MEA/Ppy/R-ZrP films. These two stable
multilayers were used as substrates for the self-assembly of CdSe
nanoparticles. The resulting mean thicknesses of the CdSe layer,
determined by ellipsometry, were 25-26 Å (in Au/MEA/Ppy/
PSS/CdSe) and 17-20 Å (in Au/MEA/Ppy/R-ZrP/CdSe),
respectively. The principle of the electrostatic binding is
depicted in Figure 5. Positively charged CdSe nanoparticles
were self-assembled by immersing Au/MEA/Ppy/PSS or Au/
MEA/Ppy/R-ZrP into an alcoholic dispersion of capped CdSe
for approximately 15 h. However, in the Au/MEA/Ppy/PSS
(32) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G.
M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
system the charge at the PSS interface, depending on the
thickness and the level of doping of the Ppy layer, could be
sufficiently neutralized to prevent further binding of CdSe
nanoparticles. Interestingly, CdSe nanoparticles, self-assembled
onto Au/MEA/Ppy/PSS, could not be derivatized by HDT.
Contacting the Au/MEA/Ppy/PSS/CdSe film with HDT resulted
in the quantitative release of CdSe nanoparticles, due to covalent
bond formation (between HDT and CdSe). Attempts to
sequentially self-assemble CdSe nanoparticles and R-ZrP onto
Au/MEA/Ppy/R-ZrP did not succeed. R-ZrP was able to form
a 12 Å thick layer on the top of Au/MEA/Ppy/R-ZrP/CdSe, but
dipping the resulting film into a CdSe suspension (for 20 h)
led to the formation of a layer characterized by a thickness of
only 7-8 Å only. Evidently, capped CdSe nanoparticles poorly
interacted with the second layer of R-ZrP, suggesting that the
role of the Ppy layer is important in balancing the charges in
these self-assembled systems.
Optical Properties and Photoluminescence of the Device
Components. Poly(3-methyl thiophene). The p-doped PMeT
film showed a strong absorption band with a maximum at 758
nm and a small shoulder at 470 nm (Figure 6). This absorption
spectrum is comparable to that reported for polythiophene
(interband transition at 420 nm and absorption above 600 nm
due to the gap states33). The band at 760 nm was characteristic
of a transition between the valence band and the bipolaron state
located at about 1.6 eV above it.26,34 Undoped films were
characterized by a strong absorption at 506 nm, while broad
band absorption centered at 352 nm (3.52 eV) indicated an
overoxidized film with an important loss of conducting properties.
Poly(pyrrole). After layering, films of Ppy (150-200 Å)
were characterized by an absorption band centered at 450-470
nm (2.75-2.64 eV) and a broad absorption in the range of 700800 nm (and beyond) originating from the gap states and
characteristic of a p-doping, respectively.8
(33) Kaneto, K.; Agawa, H.; Yoshino, K. J. Appl. Phys. 1987, 61, 1197.
(34) (a) Brédas, J. L.; Themans, B.; Andre, J. M. Synthetic Metals 1984,
9, 265. (b) Chung, T.-C.; Kaufman, J. H.; Heeger, A. J.; Wudl, F. Phys.
ReV. B 1984, 30, 702.
7854 J. Am. Chem. Soc., Vol. 120, No. 31, 1998
Figure 6. UV-visible absorbance of heavily p-doped (a); dedoped
(b); and overoxidized poly(3-methylthiophene) films (c); and chemically
p-doped 200 Å thick Ppy film (d).
Figure 7. (A) ) Visible absorption spectrum of CdSe nanoclusters
dispersed in butanol; (B) ) fluorescence spectrum of the same solution
after excitation at 465 nm; (C) ) emission spectrum collected for one
monolayer of CdSe bound to a quartz substrate coated with pellicular
R-ZrP; (D) ) emission spectrum obtained after deposition of three
sequences (HDT/CdSe) onto ITO substrate. It should be noted that the
maximum emission band is almost exactly three times higher than the
maximum measured in (C).
CdSe Nanoparticles. The TOPO capped CdSe nanoparticles
dispersed in anhydrous butanol were optically characterized
(Figure 7). The photoluminescence spectrum, obtained by
excitation at 465 nm, manifested itself in a narrow emission
band at 570 nm, in accord with previously published data,27
and had a quantum yield of 5% (calculated by using the emission
of Rhodamine B as a reference). Photoluminescence spectra
of CdSe nanoparticles were routinely used to monitor the
efficiency of the self-assembly in the different systems, particularly that based on electrostatic interactions. The normalized
emission spectrum obtained for one CdSe monolayer adsorbed
onto the quartz/AMS/Ppy(20 Å)/R-ZrP(10 Å) film exhibited a
band at 566 nm with a third of the intensity of that measured at
572 nm for the ITO/AMS/Ppy(80 Å)/(HDT/CdSe)3, indicating
that the coverage resulting from electrostatic interactions is
comparable to that observed for dithiols for one monolayer of
CdSe. The absorption spectrum of quartz/AMS/Ppy(20 Å)/
(HDT/CdSe)n with n ) 1, 2, or 3 was found to red shift with
respect to a single CdSe monolayer (from 543 to 548 nm, Figure
8).
Cassagneau et al.
Figure 8. UV-visible spectra of CdSe nanocrystallites (diameter ∼
35 Å) obtained for DeWice B [(ITO/anchoring agent (4-aminobutyldimethoxysilane, AMS)/Ppy(20 Å)/(HDT/(CdSe)3] showing the
sequential absorption increase of a first (1), second (2), and third (3)
layer of CdSe.
Figure 9. UV-visible spectra recorded after each deposition process
of a device formed by a sequence ITO/PMeT(film)/(HDT/CdSe)3/
HDT: (a) ) after electrochemical deposition of HDT onto PMeT; (b)
) followed by chemical deposition of CdSe; (c) ) a redoping at +1.4
V vs Ag/Ag+ in anhydrous ethanol (without adding electrolytes); (d)
) a second electrochemical deposition of HDT; (e) ) a chemical
deposition of CdSe; (f) ) a redoping; and (g) ) a final electrochemical
deposition of HDT.
Optical Properties of Doped and Undoped SCP Films.
The marked electronic transition which accompanies doping and
dedoping of PMeT films was used to characterize and monitor
the optimization of the layering and of the self-assembly by
absorption spectrophotometry. Self-assembling of negatively
charged polymers (PSS) or inorganic sheets (R-ZrP) on the top
of a ITO/AMS/Ppy film resulted in a dedoping, as indicated by
the appearance of a strong red color and an absorption band at
506 nm. However, when a monolayer of HDT was deposited
under an adequate potential (i.e., +1.4 V vs Ag/Ag+), the film
could maintain the initial doping, as demonstrated by optical
spectroscopy (Figure 9).
Immersion of the ITO/PMeT/HDT film in an alcoholic
dispersion of CdSe resulted in quantitative dedoping; as
indicated by the appearance of the absorption band at 508 nm
with an onset close to 600 nm (see curve a in Figure 9). The
redoping was affected by reimmersion of the Au/PMeT/(HDT/
Layer-by-Layer Assembly of Zener Diodes
CdSe)n or the ITO/PMeT/(HDT/CdSe)n films in a HDT solution
for 10-15 min (depending on the nature of the substrate).
Redoping was accomplished by BF4- ions trapped or still
remaining in the polymer. Alternatively, redoping was carried
out after each step of CdSe nanoparticle self-assembly, by
placing the film into an electrolytic bath which contained 0.05
M TBA+‚BF4- in acetonitrile (see below). Redoping was
monitored by absorption spectrophotometry by following the
growth of the band at 400-430 nm and the appearance of a
new band with a maximum at 800 nm (1.55 eV), characteristic
for p-type doping (see curves c and f in Figure 9). The blueshift of the absorption expected for an undoped DeWice A
appeared both after derivatization of the polymer thin DeWice
A and the CdSe monolayer formation when electrochemical
method is used and increased after every electrochemical
operation (see Figure 9). These films exhibited an open circuit
voltage of only +0.6 V, indicating a partial loss of doping of
PMeT. Indeed, the formation of shorter polymeric chains has
been reported to be responsible for an increase in the effective
value of the band gap.35 Based upon the observed shift of the
maximum absorption of the HOMO-LUMO transition (from
472 nm for a p-doped PMeT film to 420 nm for the ITO/AMS/
PMeT/HDT film), we can estimate that the band gap increases
about 0.3-0.4 eV after a complete electrochemical process
leading to the deposition of three monolayers of CdSe. It has
been reported that an overoxidation of the polymer occurs
around a voltage of +1.8 V (vs Ag/Ag+).36 Overoxidation at
these very positive potentials causes nucleophilic attack of water
on the polymer and results in the formation of hydroxyl,
carbonyl, quinone, and carboxylic groups.37 These reactions
also cause C-C bond cleavage and other structural defects,
which reduce the conjugation length of the polymer. An
increase in band gap is therefore indicative of shorter conjugation length.38 This phenomenon was clearly observed both when
an overpotential of +2.5 V or a potential of +1.4 V vs Ag/
Ag+ was applied to the ITO/AMS/PMeT(HDT/CdSe)n films.
However a certain reversibility in the degradation of the polymer
could still be evidenced on applying a potential of +1.4 V. This
effect was visualized by following the shift of the absorption
band located in the 400-500 nm region, depending on the level
of doping. Indeed, electrochemical deposition of a first HDT
layer onto PMeT was characterized by a band centered at 404
nm, which after deposition of a second and third layer of CdSe
shifted to 445 and 420 nm, respectively. Figure 9 presents the
evolution of the maximum absorption depending on the number
of electrochemical or self-assembling operations.
The position of the absorption maximum was observed to
correlate well with the extent of redoping of the ITO/AMS/
PMeT(HDT/CdSe)n films (See Figure 9). Ratios of absorption
maximum to absorbance ([λ(nm)]:[absorbance]) against the
number of layers deposited (or self-assembled) provided an
accurate appraisal of the efficiency of the redoping (Figure 10).
This ratio increased after every redoping for a given sequence,
but the ability for the device to be redoped decreased continuously with the number of sequences, indicating that the
electronic properties of the polymer were irreversibly modified
by an overoxidation reaction still occurring at a potential as
low as +1.4 V. Alternatively, the counteranions, still trapped
into the polymer network, became progressively released. In
fact, a final redoping at the same potential in an electrolytic
(35) Hoffmann, R. In Solids and Surfaces: a Chemist’s View of Bonding
in Extended Structures; VCH: New York, 1988.
(36) Krische, B.; Zagorska, M. Synthetic Metals 1989, 28, C257.
(37) Novák, P.; Müller, K.; Santhanam, K. S. V.; Haas, O. Chem. ReV.
1997, 97, 207.
(38) Brédas, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309.
J. Am. Chem. Soc., Vol. 120, No. 31, 1998 7855
Figure 10. Comparisons between the wavelengths of the undoped
domain in PMeT films and the corresponding absorbance as a function
of the number of repeated sequences: derivatizing layer (HDT)/CdSe/
redoping.
bath of 0.1 M TBA+‚BF4- led to a quantitative redoping, with
undoped domains characterized by an absorption band centered
at 401 nm (compared to 420 nm prior redoping), demonstrating
that both of these effects need to be considered.
Control of the Doping. DeWice A. Once poly(3-methylthiophene) film was electrochemically deposited onto a gold or
ITO substrate, contacting it with an alcoholic solution of HDT
or CdSe resulted in a partial loss of the doping. This dedoping
was alleviated by keeping the substrate (Au/PMeT) at a positive
potential during the deposition of HDT or CdSe. Alternatively,
a positive potential was applied after the formation of the HDT
or CdSe monolayer. Application of a positive potential for
several hours resulted in the dissolution of the PMeT film;
therefore, potentials were applied only for a period necessary
for redoping (typically for 20 min). The time duration of the
potential applied depended on the thickness of the polymer film.
Application of 2.5 V resulted in recovery of the initial level of
doping of the Au/PMeT/HDT/CdSe film, characterized by an
open-circuit voltage of 1.4 V.
Since the binding of HDT occurs by an oxidation reaction,
dedoping of the Au/PMeT/HDT/CdSe film is the consequence
of reduction. In any event, a p-doped PMeT film cannot be
stable in an alcoholic medium because its oxidation potential
(+1 V) lies near to the saturation potential of the DeWice A and
promotes further reduction and dedoping. To control both the
grafting of HDT and the doping of the film, the substrate
contacted with a HDT solution was always kept at a positive
potential. By applying a positive voltage, HDT was expected
to bind rapidly and without causing any dedoping. A 1.4 V
potential applied for at least 15-20 min resulted in the formation
of a thiolated layer with a characteristic thickness of 12-13 Å
when grafting on a gold substrate (whereas 15-20 h were
necessary for anchoring the same layer in the absence of an
applied potential). Similarly, a bias voltage was applied for
comparable time duration in the case of a gold or ITO substrate
having a polymer layer on the top in order to prevent quantitative
dissolving of this layer. Electrochemical deposition of a CdSe
layer did not improve the quality of the resulting monolayer (a
marked inhomogeneity of the film was observed from the
contact point of the electrode to the furthest area) nor the time
necessary to obtain it. As a consequence, a typical procedure
consisted of an electrochemical deposition of the anchoring
molecule (HDT) followed by the immersion of the Au/PMeT/
HDT/film into a CdSe dispersion for at least 2 h. Parallel with
formation of the anchoring layer partial redoping occurred (see
above).
In most cases a potential of +1.4 V was preferentially chosen
in order to avoid the application of an overpotential. This
7856 J. Am. Chem. Soc., Vol. 120, No. 31, 1998
Cassagneau et al.
Table 2
systems
medium
ITO/AMS/Ppy(50 Å)
neat EtOH
ITO/AMS/Ppy(35 Å)
5 mM HDT ethanolic solution
contacting time (min)
at +2.5 V vs Ag/Ag+
Vo.c. (V)
0
10
20
0
10
20
50
+0.55b
+0.51
+0.50
+0.57b
+0.46
+0.47
+0.46
absorbancesa
at 472 nm at 800 nm
0.0438
0.0308
0.0094
0.0316
0.0315
0.0308
0.0293
0.0398
0.0289
0.0117
0.0293
0.0200
0.0200
0.0182
ratio Abs(800)/Abs(472)
0.91
0.94
0.92
0.93
0.63
0.65
0.62
a A maximum of absorbance was observed at 472 nm in all cases, while a broad absorption band, characteristic of p-doping, appeared centered
beyond 820 nm and has been arbitrarily monitored at 800 nm. b These values have been measured in both cases immediately after depositing the
Ppy film and in neat EtOH.
potential was applied after each deposition step of a CdSe
nanoparticle layer until an open potential circuit voltage of +1.2
V was achieved (typically after 15 min) in an acetonitrile doping
solution containing 0.05 M TBA+‚BF4-. Nevertheless an
almost complete redoping of the Au/PMeT/(HDT/CdSe)3 films
could be accomplished by the application of +2.5 V overpotential in the same medium for 80 s. Longer application times
caused irreversible damage of the polymer film, with a loss of
electroactivity, as evidenced by the presence of an absorption
band at 352 nm (characteristic for an overoxidized film see curve
c in Figure 6). It should be noted that these two procedures of
redoping did not lead to detectable differences in the electroactivity of the full device.
DeWice B. A chemically p-doped Ppy layer in the Au/MEA/
Ppy/HDT films also underwent dedoping during the selfassembly of HDT. To prevent both the dedoping and the
dissolution of the polymer by long contacting time in this
solution, the anchoring layer was electrochemically deposited
at a constant potential of +2.5 V for 15 min. This, typically
led to the formation of an 11 Å thick HDT layer and an open
circuit voltage of +0.47 V demonstrating that the Ppy layer
remained in a p-doped state. It has been reported that a Ppy
film undergoes overpotential at potential more positive than
+0.65 V vs Ag/Ag+ in water.39 To quantify the effect of the
applied anodic potential on self-assembled Ppy layer, a 35-50
Å film was prepared onto an ITO/AMS substrate. The degree
of oxidation of the polymer was monitored by means of UVvisible spectroscopy, while holding the potential at +2.5 V vs
Ag/Ag+ and contacting the surface with neat ethanol or with
an ethanol-HDT electrolyte solution. From the time-dependent
absorbance data (Table 2), it is apparent that the Ppy film is
desorbed completely without overoxidation or dedoping in
ethanol solutions that contained no HDT. The film remained
doped while dissolving, as deduced from the values of Vo.c. and
the almost constant absorption ratio of 800 nm/472 nm. In the
presence of HDT, however, both the open circuit voltage and
the absorption ratio of 800 nm/472 nm decreased initially with
time and then became constant, indicating that partial dedoping
occurred. It is known that the rate of overoxidation of Ppy
increases significantly in the presence of nucleophiles.40,41 In
the present case, it appears that HDT reacts promptly at the
surface of the Ppy film, forming a passive layer that prevents
further oxidation or dedoping from occurring. This reaction
corresponds to a Michael addition of a nucleophile, generally
(39) (a) Lewis, T. W.; Wallace, G. G.; Kim, C. Y.; Kim, D. Y. Synthetic
Metals 1997, 84, 403. (b) Mostany, J.; Scharifker, B. R. Synthetic Metals
1997, 87, 179.
(40) (a) Novák, P.; Vielstich, W. J. Electrochem. Soc. 1990, 137, 1036
and 1681. (b) Schlenoff, J. B.; Xu, H. J. Electrochem. Soc. 1992, 139, 2397.
(41) (a) Beck, F.; Braun, P.; Oberst, M. Ber. Bunsen-Ges. Phys. Chem.
1987, 91, 967. (b) Novák, P.; Rasch, B.; Vielstich, W. J. Electrochem. Soc.
1991, 138, 3300.
an amine,42 thiol in our case, to a conjugated double bond
system, such as quinones produced during the overoxidation.
Since the doping level of the Ppy layer was still high after the
reaction with HDT, redoping was not attempted. In our study,
Ppy films were about 10 times less rough than the electrochemically deposited PMeT films. Concomitantly, Ppy films were
more stable and compact than their PMeT counterparts.
Subsequent contact of the Au/MEA/Ppy/HDT films with CdSe
suspension led to the deposition of a semiconducting monolayer
of nanoparticles (Vo.c. ) 0.45 V) with a high level of coverage.
The assembling procedure is schematically described in Figure
11. The relation between the absorption of CdSe nanoparticles
at 448 nm and the number of sequences (HDT/CdSe)n deposited
onto a thin layer of Ppy (20 Å) is illustrated in Figure 8. It can
be seen that the first HDT/CdSe bilayer exhibits a more intense
absorption than any of the subsequent bilayers, indicating the
more dense coverage of the Ppy polymer than the CdSe
nanocrystalline layer by HDT.
X-ray Photoelectron Spectroscopy. XPS spectroscopy was
performed on two aged multilayer systems (stored 20 months
in air): AMS/Ppy(20 Å)/(HDT/CdSe)3/HDT, DeWice A, and
AMS/PMeT(500 Å)/(HDT/CdSe)1/HDT, film B. Both films
exhibited peaks corresponding to Cd, Se, S, C, O, and Sn.
Importantly, no phosphorus peaks were detected, confirming
the effective replacement of the TOPO by HDT on the CdSe
nanoparticles during the self-assembly. By analyzing the Se
3d peaks region, it was found that a partial oxidation of Se
occurred in both cases. In DeWice B, peaks at 53.7 and 58.4
eV were assigned to the 3d level of Se atoms in CdSe and SeO2,
respectively (Figure 12). On this basis, an approximate curve
fit to the Se 3d envelope for DeWice B gave a (selenide)/
(selenium oxide) ratio of about 6/1. This value lies in the range
of oxidation level expected for TOPO-capped CdSe nanocrystallites laid down onto HDT-derivatized gold substrate43 upon
exposure to air for 1 day and demonstrates the long-term stability
of our multilayers. A close-up survey in the domain of Cd 3d
revealed the presence of two peaks originating from levels 5/2
and 3/2. No cadmium oxide44 was detected in this domain,
confirming the crucial and stable bonding of thiols with Cd
atoms. Nitrogen was not clearly identified in both films. In
DeWice A, a close-up survey in the binding energy region
expected for N 1s did not show a peak at 401 eV as expected
for doped poly(pyrrole).44 This is probably due to the low
sensitivity of N relative to Cd. It should be also noticed that
some other elements, such as Si from AMS and In from the
substrate itself, were also not detected, while obviously present.
(42) Deinhammer, R. S.; Ho, M.; Anderegg, J. W.; Porter, M. D.
Langmuir 1994, 10, 1306.
(43) Bowen Katari, J. E.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem.
1994, 98, 4109.
(44) Beadle, P. M.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir
1996, 12, 1784.
Layer-by-Layer Assembly of Zener Diodes
J. Am. Chem. Soc., Vol. 120, No. 31, 1998 7857
Figure 11. Schematic representation of the derivatization of a Ppy layer under overpotential in alcoholic solution of 1,6-hexanedithiol (HDT),
followed by the chemical deposition of (CdSe/HDT) sequences.
I ) V2 exp(-βd/V)
(1)
where d is the thickness of the device sandwiched between the
electrodes. Plotting eq 1 logarithmically led to a good straight
line (Figure 13) which permitted the fitting of the forward branch
of the Fowler-Nordheim equation (see below). If a triangular
barrier is assumed to govern the tunneling at the interfaces, the
value of the constant β can be deduced from the following
relation47
β)
Figure 12. Close-up XPS survey spectra of Se-3d core for the system
AMS/PMeT(500 Å)/(HDT/CdSe)1/HDT, DeWice B, after storage in air
for 20 months. The SeO2/CdSe ) 1:6 ratio indicates the long term
stability of DeWice B.
Fitting parameters used in curve fits of the Se 3s/S 2s envelope
were different for both samples; a shift of 0.6 e.V was measured
between the compared S 2s envelopes, indicating that sulfur
atoms were likely surrounded by two different environments
in these two systems; indeed, in DeWice A, only HDT was
present, while thiophene units were largely predominant relative
to HDT in DeWice B.
i-V Characteristics. Electrical properties of the junctions,
particularly those relating to Zener-breakdown, of DeWice A and
DeWice B were characterized by current (i) voltage (V) measurements. The Zener breakdown was generally registered from
freshly assembled films and was observed even after 1 week of
standing, substantiating the good stability of the p+-type layer.
However, a systematic investigation of the device properties
with respect to aging was not carried out. Injection of chargess
holes and electronssis believed to occur mainly by tunneling
through the interfaces between the electrodes and the hole or
electron conductors.45 The tunneling mechanism can be expressed by the Fowler-Nordheim equation which describes the
field emission tunneling current, I, as a function of the bias
voltage46
(45) Parker, I. D. J. Appl. Phys. 1994, 75, 1656.
4x2m*φ3/2
3eh
(2)
where β has the dimensions of electric field, and is related to a
“critical field” for junction breakdown. The terms φ refers to
the barrier height and m* the carrier effective mass. i-V
characteristics present two branches, in forward and reverse
direction (Figures 14 and 15). In the reverse direction, the
increase of current measured is very abrupt and do not follow
neither a Schottky nor a Fowler-Nordheim law. Such a
behavior has been observed for an asymmetric abrupt junction
like p+-n.48 In our case, the level of doping inside the polymer
is electrochemically or chemically promoted to a much higher
value than that exist in the CdSe layer. This behavior is
characteristic for the Zener breakdown and appears in a p-n
junction in which the level of doping is strongly different,48
i.e., p+-n junction in the systems (DeWice A and B) examined
here. Interestingly, when a PMeT layer was not subjected to
redoping (i.e., the multilayer assembly was prepared without
the supply of an electrochemical procedure), the breakdown was
not observed.
At a Zener junction, the electric field is maximum at the
interface between the p-and n-type material and varies as
(46) Fowler, R. H.; Nordheim, L. Proc. R. Soc. London Ser. A 1928,
119, 173.
(47) Sze, S. M. In Physics of Semiconductor DeVices; Wiley: New York,
1981.
(48) Mathieu, H. In Physique des Semiconducteurs et des Composants
Electroniques; Masson, 1996; p147.
7858 J. Am. Chem. Soc., Vol. 120, No. 31, 1998
Cassagneau et al.
Figure 13. Fowler-Nordheim plot calculated from the i-V characteristics of the system Au/p-doped PMeT film/(HDT/CdSe)3/(Al2O3)-Al
in the forward direction. The R-value of the straight line is 0.998.
Figure 14. i-V characteristics measured for an Au/p-doped PMeT
film/(Al2O3)-Al (A) and for an Au/p-doped PMeT film/(HDT/CdSe)3/
(Al2O3)-Al (B). The contact pad area was 2.5 cm2.
Figure 15. i-V characteristic obtained for (A) a sequence Au/Ppy(∼100 Å)/(Al2O3)-Al and (B) a sequence Au/Ppy(∼100 Å)/(HDT/
CdSe)3/(Al2O3)-Al. The contact pad area was 2.5 cm2.
Eo )
(
)
eNdWn
2
(V + Vr)
with Wn )
eNd d
1/2
(3)
where Nd is the donor density, Wn the length of the space charge
layer, Vd is the difference of potential between the two regions,
Vr is the reverse voltage. Thus, for Vr ) Vd
( )
Eo ) 2
eNd
1/2
‚Vr1/2
(4)
An electric force is promoted by the electric field Eo until
the binding forces between “hot” electrons and nuclei are
overcome, thus, the electrons of the valence band of the p-type
region are released toward unoccupied states of the conduction
band of the n-type region. The material becomes conductor
and the voltage Vr readily stabilizes. The limit of Vr is
depending upon both the width of the depletion layer and upon
the levels of doping in the semiconductor. The limit observed
for the reverse voltage is also called the Zener voltage Vz. The
pairs hole-electron created by ionization are drained off by the
electric field and generate an important reverse current. Electrons are directly emitted, by tunneling through the space charge
layer, from the valence band to the conduction band. The
voltage cannot increase beyond Vz, leading to the i-V characteristics reported in Figures 14 and 15. Typically, this effect is
observed for heavily doped asymmetric p-n junction, having
a very narrow space charge layer (∼ 500 Å) for Vz below five
volts.48
The Zener breakdown should appear once a potential is
reached which corresponds to that of the difference between
the conduction band edge of CdSe and the valence band edge
of the polymer. According to the energy level diagram sketched
in Figure 2, this difference is between 2.3 and 2.4 eV at the
PMeT/CdSe junction and about 2.66 eV at the Ppy/CdSe
junction. It is observed that the breakdown occurs at about -2.5
and -2.3 V for PMeT and Ppy, respectively. Thus, the energy
required for inducing a reverse current is in a fair agreement
with the minimum energy to be applied by considering Figure
2, indicating that Vd ≈ Vr (eq 4) with the Zener breakdown
taking place owing to the high density of donors Nd in the
polymer film. However, the influence of the presence of polaron
states between the donor and acceptor levels of the Zener
junction is not quantified by eq 3, for example, the distance
separating the polaron band from the valence band edge. It
can be assumed that the presence of a polaron band close to
the valence band might assist in the accommodation of extrapositive charges thus favoring the departure of electrons toward
CdSe nanoparticles as expected if the breakdown occurs at a
potential close to what we predict from the energy level diagram.
This result is quite interesting and should allow further control
of such a Zener breakdown by manipulating the nature of the
SCP layer. The i-V characteristics observed for the junction
Au/PMeT/Al2O3-Al (see curve A in Figure 14) is symmetric
and can possibly be interpreted in terms of the ability for the
polymer to accept both negative and positive polarons; this
phenomenon has been previously observed for a poly(3octylthiophene) thin layer sandwiched between ITO and Al
electrodes.33,49 However, there is still a slight ohmic behavior
between the rectifying branches, possibly related to the direct
contact with an oxidizing metal and its superficial oxide layer,
as observed for the junction Au/Ppy/(Al2O3)-Al.50
Further assembly of several sequences of HDT/CdSe onto
Au/PMeT/HDT/CdSe (DeWice A) and Au/MEA/Ppy/HDT/CdSe
(DeWice B) demonstrates, that the presence of CdSe determines
the i-V curves, with characteristics indicating, unambiguously,
that a p+-n junction has been formed. In the forward direction,
the increase of current follows the Nordheim-Fowler equation
whatever the nature of the SCP. Figure 13 shows the NF plot
obtained in the case of a junction consisting in Au/PMeT(∼800
Å)/(HDT/CdSe)3/(Al2O3)-Al. An expected Fowler-Nordheim
behavior was observed above 1.5 V in the forward direction,
with a noticeable deviation below this value. The barrier heights
to the tunneling are low, comprised between 0.025 and 0.05
eV, depending on the polymer used, in the range of energy
expected for a thermoionic formation of carriers; however, no
matching with the Schottky equation was obtained. It should
be noted that eq 2 is deduced from the flux of carriers by
(49) Garten, F.; Schlatmann, A. R.; Gill, R. E.; Vrijmoeth, J.; Klapwijk,
T. M.; Hadziioannou, G. Appl. Phys. Lett. 1995, 66, 2540-2542.
(50) Inganäs, O.; Lundström, I. Synthetic Metals 1984/85, 10, 5.
Layer-by-Layer Assembly of Zener Diodes
considering an approximate expression for the tunneling probability of a triangular barrier.51 Usually, the barrier height is
in the order of 0.1 eV, and this approximation is no longer
accurate at room temperature. The expression for the tunneling
current requires an additional temperature-dependent term.51
Such a corrected equation explains the observed deviation from
the FN law at small applied fields52 and might also account for
the present deviation (Figure 13). Additionally, the applicability
of this equation remains valid as long as β < [(1/kBT) - (1/
φ)]. If this condition is not fulfilled, thermoionic emission
dominates the current injection.
Previous studies on related organic-inorganic p-n junctions
generally involved PMeT and bulk CdS,53 Ppy and bulk CdS,54
PPV or hole transporting layer and CdSe quantum dots,7,15 and
PPV and CdS films.55 To the best of our knowledge, in none
of these cases, a Zener diode behavior was mentioned. Bulk
CdS was used with a SCP for promoting photovoltaic properties;
the valence band edge of p-doped PMeT or Ppy was lying in
vicinity of the conduction band edge of CdS (4.5 eV vs
vacuum53c); in that case a metallic-like SCP layer was deposited
onto a CdS crystalline thick layer leading to a semiconductormetal Schottky-type junction at forward bias. Observation of
a Zener breakdown in a device exhibiting good photovoltaic
properties is highly unlikely since the close vicinity of the
valence band of the SCP to the conduction band of the inorganic
semiconductor should result in charge scavenging under reverse
bias.
The electroluminescent devices based on CdSe quantum dots
and polymeric hole transporting materials exhibit a more
complicated thickness dependent i-V characteristics.14b The
current, on using poly(vinylcarbazole) and an oxadiazole
derivative, was found to be dependent on the strength of the
electric field, and a symmetrical i-V curve was observed.14b It
should be noted that these devices were found to be both nonOhmic and nonrectifying in the classical sense. On the other
hand, Colvin et al. reported a Schottky-like behavior at forward
bias for a PPV/(HDT/CdSe)n system7 with i-V curves independent of the sample thickness.56 In this case, the charge
injection was dominated by the metal-nanocrystal junction. More
(51) Koehler, M.; Hümmelgen, I. A. Appl. Phys. Lett. 1997, 70, 3254.
(52) Heeger, J. A.; Parker, I. A.; Yang, Y. Synthetic Metals 1994, 67,
23.
(53) (a) Frank, A. J.; Glenis, S.; Nelson, A. J. J. Phys. Chem. 1989, 93,
3818. (b) Glenis, S.; Frank, A. J. Synthetic Metals 1989, 28, C681. (c) Frank,
A. J.; Glenis, S. In Photochemistry and Photoelectrochemistry of Organic
and Inorganic Molecular Thin Films; SPIE, 1991; pp 50-57. (d) Nguyen
Cong, H.; Sene, C.; Chartier, P. Solar Energy Materials and Solar Cells
1993, 29, 209. (e) Nguyen Cong, H.; Sene, C.; Chartier, P. Solar Energy
Materials Solar Cells 1993, 30, 127-138.
(54) (a) Frank, A. J.; Honda, K. J. Phys. Chem. 1982, 86, 1933. (b)
Hagemeister, M. P.; White, H. S. J. Phys. Chem. 1987, 91, 150. (c)
Tsamouras, D.; Dalas, E.; Sakkopoulos, S.; Vitoratos, E. Appl. Surf. Sci.
1993, 65/66, 388.
(55) Kumar, N. D.; Joshi, M. P.; Friend, C. S.; Prasad, P. N.; Burzynski,
R. Appl. Phys. Lett. 1997, 71, 1388.
(56) Bowen Katari, J. E.; Colvin, V. L.; Alivisatos, A. P. In Biomimetic
Materials Chemistry; Mann, S., Ed.; VCH: 1996.
J. Am. Chem. Soc., Vol. 120, No. 31, 1998 7859
recently, a CdS layer was deposited by wet chemical processing
onto a PPV layer and submitted to annealing, to stimulate the
electroluminescence by improving the electron injection into
the device.55 With the PPV-based devices, the effect of
chemically depositing CdSe quantum dots or a CdS film on
the final doping of the SCP was not evaluated, and the method
of deposition of HDT onto PPV was not clearly stated.7,15,56
Furthermore, the spin-coated PPV film was obtained by the
thermal treatment of the PPV-precursor in a reducing atmosphere, which leads to undoped PPV.56 Under reverse bias, due
to the blocking nature of the electrodes and the presence of a
p-n junction, no current rectification is expected. It is likely
that depositing an n-type semiconducting layer onto a SCP layer
strongly affects the conducting properties of the p-type layer,
as it is demonstrated in this study. It is therefore essential to
maintain an appreciable level of doping in the SCP layer by an
appropriate chemical surface passivation for promoting a Zener
diode behavior. The degree of organization of the polymer also
influences the quality of the interface, as it can be seen for
systems in which the Ppy and PMeT layers had been deposited
by two different approaches (see above).
Conclusion
Details have been provided in this paper on the assembly
and self-assembly of ultrathin functioning p-n junction based
devices, capable for rectification and Zener breakdown. The
main interest in Zener breakdown is to produce “hot” electrons
in the n-type layer which could be used, for instance, for
stimulating the luminescence in a phosphor and hence to provide
a route to bimodal electroluminescence, with two distinguishable
emissions depending on the potential direction. Further works
should be devoted to the synthesis and stabilization of nanocrystalline phosphors (i.e., earth rare doped II-VI semiconductors) into similar devices. Most significantly, however, the
present work has demonstrated the versatility and power of the
self-assembly chemistry approach in nanostructuring advanced
electronic devices.
Acknowledgment. We thank the National Science Foundation (CHE-9529202) for support of this research. Instrumentation
for AFM experiments was provided by National Science
Foundation Grant CHE-9626326. T.C. thanks Vince Bojan for
helpful discussions and comments in the analysis of XPS data.
Supporting Information Available: Figure A a wide scan
XPS spectra for (a) ) the system AMS/PMeT(500 Å)/(HDT/
CdSe)1/HDT, DeWice B, and (b) ) AMS/Ppy(20 Å)/(HDT/
CdSe)3/HDT, DeWice A, after storage in air for 20 months and
Figure B a close-up XPS survey spectra of Cd 3d core for
DeWice B (a) and DeWice A (b). Note the absence of signal for
cadmium oxide (2 pages, print/PDF). See any current masthead
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